In electrophotography, the copying speed tends to increase further in recent years to meet the ever increasing quantity of information to be dealt with. In high-speed photography, there is hence the tendency that the number of copies increases further. It is desired that image quality has the same quality from the first copy to the several ten thousandth copy, to say nothing of the need for complete fixing of toner on paper sheets. It has, however, been attempted primarily to improve problems which occur after fixing of the toner on paper sheet, for example, to improve offset property and low-temperature fixing property. No substantial attention has, however, been paid to the need for uniform deposition of the toner at a constant concentration on the paper sheet in each copy. The quantity of electrostatic charge on toner is an important factor for determining the amount of the toner to be deposited on the paper sheet. It has been known that image density can be controlled by this mechanism. Triboelectric charging, however, takes place by friction between toner and a carrier, for example, in a two-component developer. Agglomerates of fine particles of a colorant such as carbon black and an undispersed charge control agent, said colorant and charge control agent being contained in the toner, often induce fouling of a carrier and a photosensitive member.
Such agglomerates and undispersed charge control agent are also responsible for troubles such as background scumming, variation of image density, and image quality due to a damaged photoconductor. It has been attempted to solve these troubles by choosing, in combination, conditions such as the temperature upon kneading, residence time and the type of screw(s) for a kneader and/or by modifying kneading conditions such as revolution speed and the dispersion method. Despite these modifications, the above troubles have not been solved yet, for example, toner varies in properties from one lot to another.
As disclosed, for example, in European Patent Publication No. 323513 previously filed naming the same inventors as in the present application, it has been found that a toner composition, which features smaller variations in the quantity of electricity charged during copying, can be provided with excellent high-speed and low-temperature fixing property by using as a binder a uncrosslinked polymer having a number average molecular weight (Mn) of 2,000-15,000 and a Z average molecular weight (Mz) of at least 400,000, the ratio of the Z average molecular weight to the number average molecular weight (Mz/Mn) being 50-600, or a mixture of the uncrosslinked polymer.
However, toner has still been observed to vary in properties from one lot to another even when the above requirements for the Z average molecular weight and the number average molecular weight are met. Differences are also observed among bags when toner is packaged especially from a storage vessel of a large capacity like a silo. It has also been observed that differences occur when the setting of the size for ground particles is changed. As is indicated in the above patent publication, inclusion of a crosslinked polymer even in a small quantity has been known to result in the drawback that the quantity of electricity to be charged varies substantially.
The present inventors thought that it would be difficult to overcome all the problems by simply modifying the conditions required upon heating, melting and kneading a colorant, a charge control agent and the like together with a binder, namely, 1) the high viscosity condition required for disintegrating agglomerates of the colorant and the charge control agent and 2) the low viscosity required for wetting surfaces of disintegrated agglomerates with the binder to improve the uniform dispersibility, in other words, to fully satisfy the flowability conditions by merely modifying mechanical conditions for the premixing stage before the kneading and those for the kneading. As a result, the properties of the binder have been found to vary from one package to another when Mz, which governs the viscosity of the binder, is made larger and Mn, which controls the flowability of the binder, is set at a particular value. Further, due to segregation of the binder, the toner has been found to include those having an unduly large average particle size and those containing too much fine powder, i.e., an excessively small average particle size.
Toner having a uniform particle size, from which large and small particles have been excluded, has heretofore been considered ideal. However, this has now been found wrong. It has hence been found that particles of a binder employed upon batchwise premixing for the production of toner are required to contain both particles on the side of larger particle sizes and particles on the side of smaller particle sizes and also that care should be exercised to avoid concentration of particles of a particular particle size due to segregation or the like.
It has also been found that a toner composition--which contains a colorant and charge control agent in a uniformly dispersed state, is free of agglomerates, has excellent electrification stability and can consistently provide pictures of excellent quality during a long-time copying operation--can be obtained by preparing the toner composition through premixing and kneading steps subsequent to adjustment of the particle size of a binder to a specific range. Although the prior art is difficult to uniformly disperse a colorant, a charge control agent and the like because the viscosity varies considerably upon kneading due to cleavage of molecules, it has also been found that a crosslinked polymer can be included in a small amount.